Backend Engineer

The Backend Engineer will design and manage services and APIs for eligibility verification, split payments, and LMN issuance, while collaborating with frontend teams and business partners to meet user needs. The summary above was generated by AI About Flex

Flex was founded to solve a simple but costly problem: making it easy for health and wellness brands to accept HSA and FSA payments online. We’re backed by First Round, Core VC, Rethink Capital power checkout for Equinox, Dermstore, Therabody, and Ultrahuman to name a few. For years, only giants like Amazon and Walmart could process these payments at checkout, leaving smaller merchants - and their customers - to deal with frustrating reimbursement paperwork.

Our technology verifies product eligibility in real time, handles split payments for eligible and non-eligible items, and can even issue Letters of Medical Necessity during checkout. With $150 billion in annual HSA/FSA spending potential, we’re giving brands the tools to unlock new revenue and making it easier for consumers to use the benefits they’ve earned., We’re looking for a Backend Engineer to design and own the services and APIs that power Flex - eligibility verification, split payments, and LMN (letter of medical necessity) issuance at checkout to name a few. You’ll work closely with frontend engineers, design, and business stakeholders to build reliable systems that move real money correctly and unlock HSA/FSA spending for merchants and consumers.

This is a role for someone who thrives in ambiguity, takes initiative, and is excited about building at an early-stage company. Responsibilities (What You’ll Do)

• Design, build, and own backend services and APIs that power Flex’s applications
• Model the domain - eligibility rules, transactions, merchants, consumer accounts, etc. - in data structures and APIs that other teams build on
• Partner with frontend, design, and business stakeholders to translate merchant and consumer needs into reliable APIs and workflows
• Own and improve our backend service architecture, data models, and integrations
• Analyze, troubleshoot, and resolve production issues rooted in business logic, data correctness, and transactional edge cases

• Collaborate on cross-functional projects that connect the full Flex experience, from consumer checkout to merchant analytics

• Create and maintain documentation for APIs, services, and on-call runbooks
• Contribute to a culture of learning, problem-solving, and operational excellence

• You have 5+ years of experience building production backend services and APIs in a typed language (Rust, TypeScript/Node, Go, Python, etc.)
• You have strong relational database fundamentals and experience designing APIs that other teams or external partners depend on
• You’re resourceful, curious, and comfortable learning new tools quickly
• You thrive in fast-paced, dynamic environments and enjoy wearing multiple hats
• You’re collaborative and enjoy working across teams to solve problems
• You have an execution mindset and always keep the end user in mind
• You’re proficient at leveraging AI tools to ship faster

Nice to Have

• Experience with building systems using Rust

• Background in payments, fintech, or health benefits (HSA/FSA), or another regulated, money-moving domain

• A mission-driven team making healthcare spending effortless
• Early-stage impact - your work will directly shape our growth and success
• Collaborative, high-trust, and transparent culture
• Competitive compensation and equity package
• Flexible, remote-friendly work environment